1. Field of the Invention
This invention relates to hydraulic vane pumps especially to vane pumps for use in an automotive power steering system. More particularly, the invention pertains to a technique for enhancing cold-start capability of the pump.
2. Description of the Prior Art
A conventional rotary vane pump includes porting that connects the output passage, where high system pressure is present, to the base of rotor slots containing radially sliding vanes. System pressure develops forces at the base of the vane forcing the vanes radially outward into contact with a cam profile. However, when a vane pump is stopped, all pressure differential within the pump and the system supplied by the pump is reduced to zero. Then, gravity, unopposed by pressure forces operating in the opposite direction, causes vanes located above the horizontal center line of the rotor to fall within the rotor to the bottom of the rotor vane slots, thereby causing loss of seal at the contact point between the vane tip and the cam profile and loss of fluid volume normally present in the vane slots below the radially inner edge of each vane. Vanes located below the horizontal center line when the pump is stopped remain in contact with the cam profile, thereby maintaining a seal between the vane tip and the cam surface and a full volume of fluid in the rotor slot below the vane.
U.S. Pat. Nos. 4,420,290 and 4,422,834 describe a power steering pump in which arcuate passages are radially and angularly aligned with the bottom of the vane slots in the rotor. These passages, however, are not connected to similar passages on the opposite axial side of the rotor but instead deliver fluid in the vanes to the outlet ports 82, 83.
Various techniques have been developed to reduce time required to force vanes outward into sealing contact with the cam profile and to improve performance particularly after cold temperature starting. U.S. Pat. No. 4,386,891 describes a rotary hydraulic vane pump in which porting formed in a thrust plate and pressure plate directs fluid from the rotor vane slots of descending vanes near the outlet port to vane slots of rising vanes adjacent the inlet ports. Each of two pairs of ports in the thrust plate are connected by a restricted passage; each pair is connected to the other pair by a nonrestricted passage. Two pairs of passages in the pressure plate are connected to the members of each pair by a highly restricted passage whose flow area is between 15% and 23% of the flow area of the restricted passage in the thrust plate. The restrictions and the passages in which they are located cause fluid flow opposite the direction of rotor rotation.